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   难度：Easy
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   <h1 class="question_title">
    204. Count Primes
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   <p>
    Count the number of prime numbers less than a non-negative number,
    <b>
     <i>
      n
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    </b>
    .
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    <strong>
     Example:
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<strong>Input:</strong> 10
<strong>Output:</strong> 4
<strong>Explanation:</strong> There are 4 prime numbers less than 10, they are 2, 3, 5, 7.
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    204. 计数质数
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   <p>
    统计所有小于非负整数&nbsp;
    <em>
     n&nbsp;
    </em>
    的质数的数量。
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    <strong>
     示例:
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   <pre><strong>输入:</strong> 10
<strong>输出:</strong> 4
<strong>解释:</strong> 小于 10 的质数一共有 4 个, 它们是 2, 3, 5, 7 。
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